1. Field of the Invention
This invention relates to a method and apparatus for improving resolution features printed on an optical mask for semiconductor fabrication, and in particular, to a method and apparatus for using a hardmask/barrier layer directly between a chrome layer and a resist layer for etching chrome films on optical masks and to improve image size uniformity and reduce errors in the nominal image size on optical masks.
2. Description of Related Art
In the manufacture of circuit patterns on electronic components, such as the manufacture of integrated circuits on semiconductor substrates, photomasks are used to transfer the desired circuit pattern onto the substrate workpiece. A photomask is typically employed a large number of times for the production of numerous electronic devices. This places stringent demands on the quality of a photomask since any flaws or defects in the photomask are reproduced in the workpiece, which directly affects the operability of the resultant workpiece.
A conventional photomask comprises a patterned metal film, such as chrome, deposited onto the surface of a transparent base and coating the metal film with a photoresist. A desired pattern is exposed and developed on the photoresist, and then portions of the underlying metal film are removed from the developed areas. Any remaining photoresist is subsequently removed leaving a patterned metal film on the substrate workpiece.
In typical processes of manufacturing a photomask on a chrome film, a wet etch process may be used in which ammonium cerium (IV) nitrate and perchloric acid are employed. However, the use of a wet etch process in the manufacture of a chrome mask makes it difficult to manufacture the mask with high accuracy as a result of the wet etching processes causing side etching effects/biases. Dry etch processes are also employed for the formation of photomasks on chrome films. A typical dry etch process of chrome masking employs the use of a mixed gas of carbon tetrachloride (CCL4) and oxygen (O2). The dry etch processes were found to be advantageous for manufacturing a mask with high accuracy, however, the etch rate of chrome using conventional dry etch processes is low. Furthermore, in conventional dry etch processes, the selection ratio of chrome to resist is poor, i.e., during the dry etch of the resist film, the underlying chrome layer etches faster than the photoresist thereby causing defects in such chrome layer.
Accordingly, such conventional dry etch processes typically cause several defects in the pattern for the photomask, such as opaque defects, clear defects, and poor resolution features, which in turn are transferred to the workpiece rendering it inoperable. Not only does the poor dry etch selectivity between the resist and chrome lead to defects on the mask, it also limits the minimum resolution that is achievable on the photomask as a result of the resist thickness needing to be increased, and thereby lowering the resolution, to compensate for the poor selectivity. Also, the poor etch selectivity between the resist and chrome films can lead to degraded critical dimension uniformity across the mask, particularly as the limits of usable resist thicknesses are approached.
A prior art solution to the poor etch selectivity between the resist and chrome films is to replace the chrome film with a silicide layer over the transparent substrate for a dry etch process. The silicide layer is formed over the transparent substrate to provide good joining ability between the mask material of silicide and the substrate, however, the use of a silicide masking material introduces several problems. For instance, suicides are more sensitive to the cleaning processes used during mask fabrication and may be degraded, i.e., removed and/or damaged, by such cleaning processes. Also, during the writing process charging distortions may be created due to the lower conductivity of the silicide film. Additionally, suicides cause defects in the sputter process as a combination of sputter targets, not a single sputter target, are used to form the silicide layer.
Another typical prior art solution to the poor resolution features, as a result of the poor etch selectivity between the resist and chrome films, has been addressed by modifying the resists to improve their etch resistance and selectivity to the underlying chrome film during dry etch processes. For example, a prior art technique to improve resolution features on the resultant mask is to reduce the thickness of the photoresist film deposited thereover the chrome film. In so doing, the dry etch time of the photoresist film is decreased causing premature thinning of the resist layer, thereby increasing the extent to which the underlying chrome layer is also etched and increasing the defects in the chrome layer. The thinning of the resist film may also lead to the resist film being completely removed during the chrome etch before the patterned images are fully transferred into the chrome layer. Other techniques of modifying the resists to improve their etch resistance and selectivity to the underlying chrome film during dry etch processes has been to modify the resist chemistry. However, it has been found that increasing etch resistance of the material can also degrade important resist performance parameters, such as, sensitivity, image quality and stability of the resist.
Still other prior art solutions include providing multiple layers over a transparent substrate in addition to the metal and resist layers for the formation of a photomask. However, such processes are both time consuming and expensive as they require additional processing steps.
Another issue that the industry faces as it begins the transition to the use of chemically amplified resists is the formation of a “foot” at the interface between the resist and a CrOxNy surface. This “foot” at the bottom of the post develop resist profile causes errors in both the nominal mask image size as well as the image size uniformity across the mask. In some cases the “foot” can also cause defects in the photomask pattern. It is believed that the formation of this “foot” is due to the presence of nitrogen in the Cr film and that this nitrogen poisons that resist at the Cr/resist interface leading to the poor image profiles in the resist after develop.
Attempts have been made in the art to find solutions that would eliminate the formation of the resist “foot.” Attempted solutions have included, for example, surface treatments with O2 plasma, isopropyl alcohol (“IPA”), hexa-methyl-disilizane (“HMDS”) or acid/base chemistries all prior to applying the resist. However, it has been found that none of such prior art solutions have been successful in elimination of the resist “foot.”
Accordingly, a need continues to exist in the art to provide an improved method and photomask material that allows for the achievement of the desired nominal image size and image size uniformity on the photomask while remaining efficient, easy and relatively inexpensive to make the requiring minimal changes to the existing materials.
According, a need continues to exist in the to provide an improved method and photomask material which allows for the achievement of a desired minimal resolution on a photomask and which eliminates the resist “foot” at the interface between the resist film and the CrOxNy surface.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved photomask material for manufacturing a photomask and an improved method for manufacturing such photomask which achieves a desired minimal resolution on a photomask.
Another object of the present invention is to provide an improved photomask material for manufacturing a photomask and an improved method for manufacturing such photomask that eliminates the resist “foot” at the interface between the resist film and the CrOxNy surface.
It is another object of the present invention to provide a photomask material and method of making such mask that is not subject to the limitations of imaging conventional chrome blocking layers.
A further object of the invention is to provide a simplified photomask material and method of making such mask that is less sensitive to mask defects.
Another object of the invention is to provide a photomask material and method of making such mask that is relatively easy and inexpensive to manufacture using existing tools and processes.
It is yet another object of the present invention to provide a photomask material and method of making such mask that will lead to improved critical dimension uniformity of the photomask.
Yet another object of the present invention is to provide a photomask material and method of making such mask that will lead to improved control of the nominal image size.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.